The progression of President Trump’s reaction to the latest reported chemical-weapons attack on the Damascus suburb of Douma is less straight line than seesaw. On Sunday, the president warned Syrian Bashar al-Assad’s regime and its backers in Russia and Iran that there would be a “big price to pay” for the assault, in which dozens of people, including children, were killed. The next day, he hedged: Trump announced the U.S. would make “some major decisions” on how to respond...